Hadith Qudsi 11
An example of Allah's Mercy on his slaves
11) Narrated Abu Huraira that Allah's Messenger said: Allah says: Whenever My slave intends to do a bad deed, (I say to the deeds recording angels): Do not record it against him until he (actually) commits it. If he has done so, write it down exactly as one in his record book. But if he refrains from it for My sake, write down this as a virtue in his favour. And when he intends to do a good deed, but does not actually do it, write it as a virtue for him. And if he puts it into practice, write its reward equal to, from ten to seven hundred times (in his account).
(This Hadith is reported by Bukhari, Muslim and Tirmidhi).
This above Hadith briefly describes that Allah, if He likes, can forgive even the most sinner person believing in Oneness of Allah and the Prophethood of Muhammad But it doesn't mean that everyone should take it as a cover and involve himself intentionally in evils relying on the Mercy of Allah. A good Muslim who has in his heart fear of Allah, should perform righteous deeds and avoid sins as per commands laid down in the Qur'an and Hadith.